fromJsonSchemaDocument: import the not keyword #10044

What is the problem this feature would solve?

On 4.0.7, importing a schema that uses not fails:

SchemaRepresentation.fromJsonSchemaDocument(JsonSchema.fromSchemaDraft07({
  type: "object",
  properties: { a: { type: "string" } },
  not: { required: ["b"] }
}))
// Cannot import JSON Schema keyword "not": Effect has no equivalent constraint.

github-workflow.json uses not 9 times and dependabot-2.0.json once, mostly to forbid combinations such as branches together with branches-ignore.

Context: validating .github/dependabot.yaml and GitHub workflow files with SchemaRepresentation.fromJsonSchemaDocument against the SchemaStore documents dependabot-2.0.json and github-workflow.json, to replace ajv. Since #7410 the importer rejects keywords it cannot represent, which is the right failure, but it means both real-world documents cannot be imported at all. This is one of five gaps found in those two documents; each is filed separately.

What is the feature you are proposing to solve the problem?

Import not as a check that fails when the value decodes against the negated schema. Exporting it back to JSON Schema as not would keep round-trips lossless.

What alternatives have you considered?

• Stripping not with onEnter: imports, but silently accepts the invalid combinations the keyword exists to reject, which is what fromJsonSchemaDocument silently ignores not, if/then/else, dependencies, and contains #7410 set out to prevent.
• Keeping ajv for these documents: what we do today.